Mustelidae or Mustelids (from Latin mustela, weasel), commonly referred to as the weasel family, is a family of carnivorousmammals. The Mustelidae is a diverse family and the largest in the order Carnivora, at least partly because it has in the past been a catch-all category for many early or poorly differentiated taxa.[1]

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The Mustelidae in general are phylogenetically relatively primitive and so were difficult to classify until genetic evidence started to become available. The increasing availability of such evidence may well result in some members of the family being moved to their own separate families, as has already happened with the skunks, previously considered to be members of the mustelid family.

Mustelids vary greatly in size and behavior. The least weasel is not much larger than a mouse. The giant otter can weigh up to 76 lb (34 kg). The wolverine can crush bones as thick as the femur of a moose to get at the marrow, and has been seen attempting to drive bears from kills. The sea otter uses rocks to break open shellfish to eat. The marten is largely arboreal, while the badger digs extensive networks of tunnels, called setts. Some mustelids have been domesticated. The ferret and the tayra are kept as pets, or as working animals for hunting or vermin control. Others have been important in the fur trade. The mink is often raised for its fur.

As well as one of the most species-rich families in the order Carnivora, mustelidae is one of the oldest. Mustelid-like forms first appeared about 40 million years ago, roughly coinciding with the appearance of rodents. The direct ancestors of the modern mustelids first appeared about 15 million years ago.

Within a large range of variation, the mustelids exhibit some common characteristics. They are typically small animals with short legs, short round ears, and thick fur. Most mustelids are solitary, nocturnal animals, and are active year-round. [2]

Mustelids, with the exception of the sea otter,[3] have anal scent glands that produce a strong-smelling secretion the animals use for sexual signaling and for marking territory. The most developed of these scent glands are found in skunks (Mephitinae), which were moved into a new family, Mephitidae, following DNA analyses [4].

Most mustelid reproduction involves embryonic diapause. The embryo does not immediately implant in the uterus, but remains dormant for a period of time. No development takes place as long as the embryo remains unattached to the uterine lining. As a result, the normal gestation period is extended, sometimes up to a year. This allows the young to be born under more favorable environmental conditions. Reproduction has a large energy cost and it is to a female's benefit to have available food and mild weather. The young are more likely to survive if birth occurs after previous offspring have been weaned.

Mustelids are predominantly carnivorous, although some will sometimes eat vegetable matter. While not all mustelids share an identical dentition, they all possess teeth adapted for eating flesh, including the presence of shearing carnassials. Although there is variation between species, the most common dental formula is[2]:
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Several members of the family are aquatic to varying degrees, ranging from the semi-aquatic mink, the river otters, and the highly aquatic sea otter. The sea otter is one of the few non-primate mammals known to use a tool while foraging. It uses "anvil" stones to crack open the shellfish that form a significant part of its diet. It is a "keystone species," keeping its prey populations in balance so some do not outcompete the others and they do not destroy the kelp in which they live.

The black-footed ferret is entirely dependent on another keystone species, the prairie dog. A family of four ferrets will eat 250 prairie dogs in a year. The ferrets require a prairie dog colony of 500 acres (2 km²) to maintain a stable population to support their predation.

Several mustelids, including the mink, the sable (a type of marten) and the ermine (stoat), boast exquisite and valuable furs and have been accordingly hunted since prehistoric times. Since the early middle-ages the trade in furs was of great economic importance for northern and eastern European nations with large native populations of fur-bearing mustelids, and was a major economic impetus behind Russian expansion into Siberia and French and English expansion in North America. In recent centuries fur farming, notably of mink, has also become widespread and provides the majority of the fur brought to market.

One species, the sea mink (Neovison macrodon) of New England and Canada, was driven to extinction by fur trappers around the same time that the passenger pigeon was declining in the late 19th century. Its appearance and habits are almost unknown today because no complete specimens can be found and no systematic contemporary studies were conducted.

The sea otter, which has the densest fur of any animal, narrowly escaped the fate of the sea mink. The discovery of large populations in the North Pacific was the major economic driving force behind Russian expansion into Kamchatka, the Aleutian islands and Alaska, as well as a cause for conflict with Japan and foreign hunters in the Kuril Islands. Together with widespread hunting in California and British Columbia, the species was brought to the brink of extinction until an international moratorium came into effect in 1911.

Today some mustelids are threatened for other reasons. Sea otters are vulnerable to oil spills and the indirect effects of overfishing; the black-footed ferret, a relative of the European polecat, suffers from the loss of American prairie; and wolverine populations are slowly declining because of habitat destruction and persecution.

One mustelid, the domestic ferret (Mustela putorius furo), has been domesticated since ancient times, originally for hunting rabbits and pest control. In recent years its popularity as a household pet has increased.